Conventionally, an air conditioning device for a vehicle heats an inside of a vehicle compartment by using a heat pump cycle. In such an air conditioning device for a vehicle, condensed water on an exterior heat exchanger freezes to form frost when a temperature of refrigerant flowing through the exterior heat exchanger falls below a dew-point temperature. If the frost is formed on the exterior heat exchanger, an amount of heat absorbed from outside air in the exterior heat exchanger reduces, which significantly degrades heating performance.
Patent Literature 1 discloses an air conditioning device for a vehicle in which an opening degree of an expansion valve is increased to increase a temperature of refrigerant in an exterior heat exchanger to thereby delay frost formation in the exterior heat exchanger when the frost formation is likely to occur in the exterior heat exchanger.